Human rights groups on Wednesday expressed concern for a group of handcart laborers allegedly beaten by security forces at the border town of Poipet.

Some 200 laborers, who ferry goods across the Thai border and back, demonstrated at the Poipet checkpoint, demanding that customs officers there drop fees and other charges.

Clashes ensued with local military police, including the firing of live rounds into the air, leaving at least four people injured.

Rights workers say the military police instigated the violence, but authorities say the demonstrators had started throwing rocks at police before the crackdown. Protesters also attempted to storm the customs office at the border in the ensuing fracas.

Ny Chakrya, head of the legal aid section for the rights group Adhoc, said military police action there was a “strong violation of human rights.” Authorities stomped on the hands of protesters and beat them unconscious before handcuffing them, he said.

Chea Da, military police commander at Poipet, said his forces had merely removed protesters from blocking the road, following hours of demonstrations that had blocked border crossings.

His soldiers fired in the air only after rocks were thrown at them and to prevent protesters from entering the customs building, he said. “If they got into the building, it would not have ended, and there would have been casualties,” he said.

Chea Da and Chhuon Hai, head of the Poipet customs office, said they were investigating the incident to find who among the protesters had started the violence.